Explosives and Blasting-Part 2

Any explosive needs stimuli like shocking, friction or flaming for it to blast, or the reaction to initiate in it. The devices used to carry out these operations are known as initiating devices.

Detonators

Initiator is a term that is used in the explosive industry to describe any devise that may be used to start a detonation or deflagration. Devices that are used to initiate high explosives are called Detonators and devises that start burning or deflagration are called squibs or igniters.

Detonators are used for initiating high explosives and contain small amounts of a sensitive primary explosive. Although they are manufactured to absorb a reasonable amount of shock during handling and transportation, they should not be abused.

In general detonators consist of an ignition charge, intermediate charge, and a base charge. Each charge in the train is selected and used to transition from heat to shock.

Instantaneous detonator

This is simpler in construction and made of an aluminum shell closed at bottom and open at the other end. It is used under dry and non-gassy conditions and initiated by the safety fuse that is inserted in its open end and crimped.

Delay detonators

Electric delay detonators

These are manufactured as two varieties – Long/half second delay detonators and short/millisecond delay detonators. These detonators are longer in length than the instantaneous electric detonators as a delay element is incorporated in between the primary charge and the fuse head. Long delay detonators are available in 0–15 numbers, with a nominal half-second time interval between each delay. In short delays the delay interval is much shorter. These types of detonators are available in a wide range of intervals using no. 6 and no. 8 strength caps. These short delays can be further classified as normal and non-incendive delays. The normal detonators are available in the range of 18–38 delays each interval varying from 8 to 100 or more milliseconds. The leg wires’ length is in the range of 4 to 60 ft. These delays are widely used in mines other than u/g coalmines. The non-incendive types of detonators are used in coalmines and are made of copper tubes with copper leg wires of 4–16 ft lengths. These are available in 10 delays with interval of 25–75 milliseconds. Both types can be used in wet conditions.

Electronic delay detonators

An electronic delay detonator is very recent and in its exterior appearance it looks like a conventional detonator. The detonator is marked with delay period number from 1 to 250. This number does not indicate delay time but only the order in which the detonators will be fired. Each detonator has its own time reference, but the final delay time is determined through the interaction between the detonators and the computerized blasting machine before their firing. In this system detonators react to the dedicated blasting machine eliminating risk of unintentional initiation by any other energy source. In the manufacturing of this detonator several elements are used as the chain reaction of igniting the detonator, and detonating the charge in the drill hole. Each element involves a time delay, which is not the same for all normally equal detonators. The reason is that each element has a certain amount of catter time.

Non-electric delay detonators: detonating relays (ms connectors)

This system is used in conjunction with detonating cords (DC) for blasting large number of holes and is capable of introducing millisecond intervals (delays) between holes or rows of holes. The system finds its applications in surface mines and u/g metalliferous mines.

Primadet and anodet non-electric delay blasting system

To safeguard against the static charge and current hazards from the electric detonators. It consists of three components:

A blasting cap (no. 6) with delay elements (short or long delay). Short delay system with 30 delay periods ranging from 25 to 250 milliseconds.

A detonating cord having PETN of 4 grains/ft. called primaline. One end of which is crimped into the blasting cap at the time of its manufacturing. It is available in different lengths 2 m to 15 m (6–50 ft).

A plastic ‘J’ connector for readily attaching the free end of the primaline to the trunk line.

Anodet is similar to primadet and it has been developed by CIL for its use during charging ANFO pneumatically in the blastholes of 25 to 70 mm. The primaline is known as Anoline in this system. In this system to locate the primer centrally in the hole the manufacturer also supplies a plastic cap holder.

The nonel system

It is an invention by Nitro Nobel AB Sweden used as a nonlectric system without use of detonating cords. The manufacturer as a standard pack of the following four components supplies it:

a Nonel tube – it is a transparent tube having 3 mm external dia. with 1.5 mm. bore. Inside wall of this tube is coated with low concentration of explosive powder that posses the ability to conduct a shock wave at constant velocity.

a plain detonator with a delay element.

a connecting block, provided with a mini-detonator – which supplies the shock wave to the Nonel tube.

a starting gun and Nonel trunk line.

A special gun initiates the complete circuit that energies a Nonel trunk line, which in turn initiates each connecting block connected to it. The mini-detonator in the connecting block supplies the shock wave. It travels through the tube and emerges in the detonator as intensive tongue of flame. The Nonel detonators are supplied in the range

of 20 delay intervals each of 25 milliseconds, and six more each of 100–150 milliseconds.

Nonel is a closed system. Each hole is supplied with a separate Nonel unit and a simple manual operation connects each unit to the preceding one. The ignition impulse, once ignited, is transmitted from unit to unit via the connecting blocks.

Several rounds may be fired in parallel. Since the system is non-electric, no balancing or instrument checking is required.

The system virtually eliminates accidents common with electrical blasting system while at the same time radically simplifies the blasting operations.

Advantages of short delay blasting

Advantages of the blasting with the use of short delay millisecond detonators comparing

the same with half second or instantaneous delays are as under:

Reduction in ground vibrations

Reduction in air concussion

Reductions in over-break

Improved fragmentation

Better control on fly rock.

FUSE/CORD SYSTEM

Safety Fuse

Uses – To initiate Non-Electrical Detonators or Ordinary Detonators

Ingredients:Black Powder

Properties:Safety fuse is a length of strong, flexible, rope-like product with a black powder core. It is designed to be initiated with a specialty designed fuse lighter or match, burn its length at a predetermined rate, and initiate a non-electric detonator. Safety fuse normally burns at a rate of approximately 35- 45 sec/ft.

Construction:Safety fuse is constructed with various types/quantities of natural and man made fibers and plastic. Safety fuse can usually be identified by its color, wax finish and other manufacturing characteristics observed on its exterior. However, with the use of colored plastics as a final covering, it may be misidentified.

Detonating fuse/cord (DC)

Detonating fuse is a cord having a primary explosive, such as PETN, as its core and warping of textile fibers, wire and plastic coverings around this core. Its VOD is around 6500 m/sec.

DC is safe to handle, extremely water resistant and capable of transmitting energy of a detonator to all points along its length. With DC detonators are not required to be put inside the holes. Some of blasting powders like ANFO requires a greater initiating effect throughout its charge column, and DC can fulfill this requirement very well. It can be initiated by using a plain or electric detonator. To blast number of holes, the DC is inserted into the holes by lacing it to a primer cartridge or threading through a cast booster. The DC coming out from each of the holes (as a branch line) is connected to a common trunk line by strapping (taping), clove hitch or by a plastic connector.

Igniter cords (IC)

It is cord-like in appearance and when ignited the flame passes along its length at a uniform rate. They can be used in surface mines, non-gassy and metalliferous underground mines, and tunnels for lighting any number of safety fuses in a desired sequence. IC connectors are required to use this cord.

BLASTING ACCESSORIES

EXPLODERS

These are the machines designed to fire the electric detonators. These machines can be classified as Generator (magneto)type and Condenser discharge type. The generator type exploder works on the principle of an electric generator through which the current can be generated. The current generated is used to fire the blasting caps connected in a circuit. In these types of exploders until certain minimum pre-fixed voltage is generated, it is not transmitted to the external blasting circuit, to avoid any misfire due to insufficient current (electric power).

These exploders are manually operated so that power can be generated any time, but require, skill handling and use. Their repair is simple and these are useful to fire multi shots.

Condenser discharge type of exploders are designed for multi shots firing. Their basic source is either a low voltage dry cell battery or an electromagnetic generator. When a low voltage battery is used, first of all, the low voltage is converted to high voltage through DC to DC converter. The high voltage so generated charges the capacitor. When capacitor is fully charged a neon lamp indicates it. The voltage is discharged to the external blasting circuit connected to the exploder. One of its drawbacks is that the voltage from the capacitor are not fully discharged to the external circuit and some residual voltage remain in the capacitor, which in turn, may fire another circuit accidentally. The peak current can become high if few shots are fired, thereby, causing the fuse head explosion and side burst of the detonators.